1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device having a semiconductor substrate and a resistor electrically connected to the substrate.
2. Description of the Prior Art
FIG. 3 shows a circuit diagram of a resistor load-type SRAM as a semiconductor device of the type described above. An SRAM 11 shown in FIG. 3 consists of FETs 12 and resistors 13.
FIG. 1 shows a sectional view of a first conventional example of the SRAM 11. In this example, an SiO.sub.2 layer 15 is formed on an Si substrate 14. A diffusion region 17 is formed on an Si substrate 14. A diffusion region 17 is formed in the substrate 14 by diffusion through an opening 16. formed in the SiO.sub.2 layer 15. The diffusion region 17 acts as the drain for the FET 12. A polycrystalline Si layer 18 is formed to cover the diffusion region 17 and the SiO.sub.2 layer 15. The polycrystalline Si layer 18 acts as the resistor 13.
The stand-by current of the SRAM 11 is determined by the resistance of the resistor 13. In the above conventional example, the polycrystalline Si layer 18 serving as the resistor 13 has a relatively low resistivity of 5.times.10.sup.5 .OMEGA..cm. Therefore, the stand-by current of the SRAM 11 is relatively high, and power consumption is high. These problems become particularly noticeable when a number of SRAMs 11 are integrated.
FIG. 2 shows a second conventional example of the SRAM 11 as an improvement over the first example. The SRAM 11 of this example is substantially the same as that of the first example example except that a polycrystalline Si layer 19 containing oxygen is used in place of the pure polycrystalline Si layer 18.
As disclosed in issued Japanese Pat. No. 13426/80, a polycrystalline Si containing oxygen has a very high resistivity: about 2.times.10.sup.6 .OMEGA..multidot. cm at an oxygen content of 2% and about 10.sup.10 .OMEGA..multidot. cm at an oxygen content of 20%. A semiconductor device having a layer of such a polycrystalline Si containing oxygen formed on a semiconductor substrate is also described in issued Japanese Pat. No. 2552/78.
The polycrystalline Si layer 19 containing oxygen is formed by a low-pressure CVD method using, for example, SiH.sub.4 (flow rate: 50 cc/min), N.sub.2 O (flow rate: 2 cc/min), and N.sub.2 or He as needed. In this case, in order to stabilize the N.sub.2 O flow rate, only N.sub.2 is supplied for 1 to 2 minutes before SiH.sub.4 is supplied. Therefore, the Si substrate 14 is oxidized, and a thin SiO.sub.2 layer 20 is formed between the diffusion region 17 and the polycrystalline Si layer 19 containing oxygen.
When P or As is doped in a connection portion between the diffusion region 17 and the polycrystalline Si layer 19 in order to reduce the connection resistance of this connecting portion, the SiO.sub.2 layer 20 becomes PSG or AsSG. Even if annealing is formed, the SiO.sub.2 layer 20 remains unchanged. Therefore, it is very difficult to ohmically connect the diffusion region 17 and the polycrystalline Si layer 19.
As can thus be seen, it is difficult to obtain a sufficiently high resistance with the pure polycrystalline Si layer 18 while it is also difficult to ohmically connect the polycrystalline Si layer 19 and the diffusion region 17 with the polycrystalline Si layer 19 containing oxygen.